8_Ch19_Steel

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CHAPTER 19

Structural Steel Construction Introduction

Structural Steel Construction

Introduction

The structural frame of a steel building is constructed by assembling prefabricated steel components at the site.

This is in contrast to the buildings whose structural system consists of load-bearing masonry or cast-in-place reinforced concrete.

In these buildings, almost the entire structure is site-constructed with virtually no prefabrication.

Therefore, issues related to component fabrication, component erection, and component assembly are important in a steel structure.

Preliminary Layout of Framing Members


Preliminary Layout of Framing Members (Design Coordination)

Preparing the preliminary framing layout of a steel-frame building (and also of wood or concrete buildings) is not simply a structural engineering exercise.

It involves careful integration of structural considerations with several nonstructural considerations, such as HVAC, the building envelope, fire resistance, interior finishes, aesthetics, and cost.

Because of its integrative nature, preparation of the preliminary framing layout is at the heart of an architect’s expertise and is generally undertaken during the design development phase.

Preliminary Layout of Framing Members (Framing Plans)

The framing layout of a structure is usually prepared in plan view; therefore, it is referred to as a framing plan.

Generally, a roof framing plan is different from a floor framing plan because of the difference in floor and roof loads.

In preparing the framing plans, establishing the structural grid is the first step.

A structural grid determines the column locations, which are generally placed at the intersection of the grid lines. Therefore, the grid lines are also called column lines.

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Preliminary Layout of Framing Members (Framing Plans) – cont’d

An important feature of the framing of a floor or roof of a steel building is that all components are one-way components, that is, they span along one direction only.

This is in contrast to cast-in-place concrete slabs, which may either be one-way or two-way slabs.

Preliminary Layout of Framing Members– cont’d (Use of W-Sections)

For the primary and secondary elements in most buildings, the choice is between the use of W-sections or steel joists, as indicated by the following four alternatives: W-sections for both primary (girders) and secondary

(beams) elements

W-sections for primary elements and steel joists for secondary elements.

Steel joist girders or trusses for primary elements and steel joists for secondary elements.

Preliminary Layout of Framing Members– cont’d (Use of W-Sections)

In some buildings, such as those with load-bearing walls, the secondary supporting elements may be omitted and the roof deck may be supported directly on the primary elements.

Preliminary Layout of Framing Members(Use of W-Sections) – cont’d

A floor structure with W-sections for primary and secondary framingelements. Note the hierarchy in the load transfer mechanism. The floor load is transferred from the deck panels (tertiary elements) to secondary elements (secondary beams). From the secondary beams, the load is transferred to the primary elements (primary beams) and then to columns. Instead of using the terms primary beam and secondary beam, the terms girder and beamrespectively,are more common. The term girder is used for a beam that supports other beams.

Preliminary Layout of Framing Members– cont’d (Use of Steel Joists)

A roof structure with steel joist girders as primary framing elementsand steel joists as secondary elements. Note that the roof load is transferred from deck panels to joists, from the joists to joist girders, and then to columns.

Preliminary Layout of Framing Members(Use of Steel Joists) – cont’d

In this roof structure, the roof deck panels transfer the load to steel joists, which transfer the load to a masonry wall. Thus, the joists function as primary elements and the deck panels as secondary elements. Observe the daylight filtering through the deck. This is because the deck is a perforated (acoustical) deck.

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The structural framing of a building must consider both gravity and lateral loads.

Therefore, simultaneously with the preparation of the floor and roof framing plans, the architect (in consultation with the structural engineer) should determine how to provide lateral load resistance to the structure.

Preliminary Layout of Framing Members– cont’d (Lateral Load Resistance)

In a steel-frame building, the choice is between the use of the following alternatives or their combinations: Rigid frames

Braced frame

Reinforced concrete shear walls

Steel shear walls

Preliminary Layout of Framing Members(Lateral Load Resistance) – cont’d

Preliminary Layout of Framing Members– cont’d (Examples of Framing Layouts)

Preliminary Layout of Framing Members(Examples of Framing Layouts) – cont’d

Preliminary Layout of Framing Members(Examples of Framing Layouts) – cont’d

Bolts and Welds


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Bolts and Welds

Steel bolts are of two types: Unfinished (or common or ordinary) bolts are made from carbon steel

and generally have the same stress-strain characteristics as A36 steel. As per ASTM specifications, they are classified as A307 bolts.

The use of A307 bolts has decreased significantly since the introduction of high-strength bolts.

High-strength bolts are based on ASTM specification A325 or A490.

A325 bolts are made from heat-treated carbon steel and have an approximate yield stress of 85 ksi.

A490 bolts are made from a heat-treated steel alloy and have a yield strength of 120 ksi.

A325 bolts are more commonly used because they cost less.

Bolts and Welds – cont’d(Snug-Tight Connection)

A typical spud wrench for making snug-tight bolted connections.

Bolts and Welds – cont’d(Slip-Critical Bolted Connection)

Steps in making a slip-critical connection using the twist-off bolt method.

Welding is a process by which connected steel parts are brought to a plastic or fluid state through heating of the parts, allowing them to fuse together, generally with the addition of another molten metal.

Welding may be done using either gas welding or arc welding.

Bolts and Welds – cont’d(Welds)

Welding advantages over bolting: Larger range of applicability

Elimination of bolts and connection gusset plates, which can amount to a substantial saving of steel in some structures.

Continuity between the connected members, which is more difficult to obtain through bolting.

Bolts and Welds – cont’d(Welding versus Bolting)

Welding disadvantages: Greater level of skill required.

Preferably carried out under the controlled conditions of a shop.

Members to be welded must be dry and free from dirt and grease.

Therefore, field welding is generally avoided as much as possible.

Bolts and Welds (Welding versus Bolting) – cont’d

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Bolting advantages over welding: Rapid

Less-skilled labor

No surface cleaning

It is more suited to field conditions than welding because weather conditions have relatively little effect on bolting.

Bolts and Welds (Welding versus Bolting) – cont’d

Connections Between Framing Members


Connections Between Framing Members

The more commonly used connection types are: Column-to-beam connections

Beam-to-beam connections

Column-to-column connections (i.e., column splice)

Connections Between Framing Members – cont’d (Typ. Type II Connections)

Typical column-beam shear (Type II) connection.

Connections Between Framing Members (Typ. Type II Connections) – cont’d

Typical column-beam shear (Type II) connections.

Connections Between Framing Members (Typ. Type II Connections) – cont’d

Typical column-beam shear (Type II) connection.

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Connections Between Framing Members – cont’d (Column-Beam Connections)

The American Institute of Steel Construction (AISC) divides column-beam connections as follows: Rigid connection, also called moment connection or AISC

Type I connection

Simple connection, also called shear connection or AISC Type II connection

Semirigid connection, also called AISC Type III connection

In practice, however, most connections are either simple or rigid connections.

Connections Between Framing Members– cont’d (Rigid Column-Beam Connections)

Typical column-beam rigid connection.

Connections Between Framing Members(Rigid Column-Beam Connections) – cont’d

Typical column-beam rigid connection.

Connections Between Framing Members– cont’d (Girder-Beam Shear Connections)

Typical girder-beam (shear) connection.

Connections Between Framing Members(Girder-Beam Shear Connections) – cont’d

Typical girder-beam (shear) connection.

Connections Between Framing Members – cont’d (Column Splices)

Column splices are a necessity in a multistory building because of the limited length of steel members.

Generally, column splices are provided every two stories.

In a three-story building, however, the columns can be continuous—without splices.

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Connections Between Framing Members (Column Splices) – cont’d

Typical column splices. Column ends are milled smooth giving full end bearing to achieve proper transfer of loads.

Steel Detailing and Fabrication


Steel Detailing and Fabrication

Unlike masonry and site-cast concrete construction, in which various components, such as walls, columns, and beams, are constructed at the site, structural steel components are brought to the site in a prefabricated and finished state, ready for erection and assembly.

There is very little cutting, notching, and drilling of components at the site.

Steel Detailing and Fabrication– cont’d (Detailing)

For most buildings, the design and detailing of connections between components (such as those shown in the previous slides) are prepared by the fabricator based on the framing plans and other information provided in the project’s structural drawings.

Leaving connection detailing to the fabricator allows the fabricator to use the details that are most economical and best suited for erection, scheduling, and the peculiarities of the site.

Where details are aesthetically significant, the fabricator must conform to the architectural and structural requirements of the project.

Steel Detailing and Fabrication– cont’d (Ordering Materials)

After the shop drawings are completed, they are reviewed first by the general contractor and then by the project architect and structural engineer.

Because the modifications expected from the review process are generally minor, the fabricator orders the structural steel sections from the rolling mills while the review of shop drawings is in progress.

Steel Detailing and Fabrication– cont’d (Fabrication)

Most large steel-fabrication shops are semiautomated, that is, computer assisted, which makes the fabrication process not only precise but also quick.

Welding in most fabrication shops is carried out manually.

For long, continuous lines of welds or repetitive welding, an automated welding system is used.

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Steel Detailing and Fabrication(Fabrication) – cont’d

A computer-aided plate and angle punching machine cuts plates and angles to the required size and drills holes of the required diameters and spacings without any manual labor.

Steel Detailing and Fabrication(Fabrication) – cont’d

A computer-aided band saw cuts heavy steel members to size. The machine is versatile enough to make skew cuts instead of a right-angle cut, as shown here.

Steel Erection


Steel Erection

The erection of the structural steel frame at the site may be performed by the fabricator or by a separate erection company.

In most cases, the general contractor will seek separate bids for fabrication and erection.

If the fabricator, selected on the basis of a competitive bid for fabrication, also submits a competitive bid for erection, the fabrication and erection may be done by the same organization. If not, the erection contract may be given to a different company.

Fabrication and erection by the same entity is obviously preferred.

Steel Erection – cont’d

The erection of a steel frame begins with the erection of columns. The columns, with their shop-welded base plates, are anchored to the bolts embedded in the foundations by the general contractor as per the fabricator’s details. In a multistory building, each column shaft is generally two stories tall.


Some of the various types of cranes used in steel frame erection.

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Leveling a column base plate using leveling nuts.


Leveling a column base plate using high-strength plastic shims placed under a thin steel plate.


A steel erector (with his body harness tied to the beam on which he sits) maneuvers the new beam into position, initially using the rope attached at the end of the beam and later by hand. The rope is referred to as a tagline in erection vernacular.


Generally, members at a connection are forced to align. In this image, a drift pin has been used to make the alignment. In some connections, crow bars or hammers may have to be used.

PRINCIPLES IN PRACTICE:

Fundamentals of Frame Construction


PRINCIPLES IN PRACTICE:Fundamentals of Frame Construction

POST-AND-BEAM CONSTRUCTION

(a) A typical post-and-beam structure used by ancient Egyptians, Greeks, and Romans. (b) Stonehenge (United Kingdom, approximately 2500 to 1700 BC) is generally cited as the earliest surviving example of the post-and-beam structure.

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Under gravity loads, the beam in a post-and-beam structure is subjected to bending, which causes it to rotate at the joint. Because of the absence of a connection between the post and the beam, the rotation of the beam at the joint is not transferred to thepost. The post is, therefore, subjected to compression only.



Under the action of both in-plane and out-of-plane lateral loads, a post-and-beam structure is unstable.


PIN-CONNECTED CANTILEVERED FRAME

A single-bay, pin-connected frame in which the beam and column are connectedtogether with a single nail, screw, or bolt. A pin connection is also referred to as ashear connection because the gravity load from the beam is transferred to the column, creating shear in the nail, screw, or bolt.



A connection between a stud and the top (or bottom) plate of a wood light frame, typically using two or three nails, is considered a pin connection.


(a) A glass roof at Toronto Airport supported by three-pin frames. Each frame consists of two members, a linear trussed member and a curved hollow-web member. The members are joined at the top with a pin connection. (b) A pin connect-ion has also been used to support each member at its support (base). All three pins in each frame have been detailed as true pins.




In a conceptual line diagram of a pin-connected frame, a pin is represented by a circular dot, and the frame members are represented by single heavy lines.

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(a) A typical isolated footing for a steel column. (b) A small column footing is regarded to provide a pinned column base because it tends to rotate under the action of the lateral load. (c) To obtain cantilever behavior from a column with an isolated footing, the footing must be large and heavy.


PIN-CONNECTED BASE FRAME

A four-pin frame is not a structure but a mechanism because it is unstable under both lateral loads and asymmetrical gravity loads.



A four-pin frame can be stabilized by using X-bracing. Note that only one of the two X-braces is active under a lateral load.



K-braces and eccentric K-braces.



(a) If a single brace is used, it must be sufficiently heavy against buckling under compression. (b) Shear wall bracing.


RIGID FRAME

(Unlike a pin connection, a rigid connection retains its 90° angle between the connected members on deformation. Consequently, a rigid frame resists gravity loads by creating bending in all three frame members. Because bending is structurally inefficient, a rigid frame is generally less economical than a comparable braced frame. Note that a rigid frame under gravity loads experiences an outwardthrust in column bases similar to that of an arch. In a single-line diagram of the frame, a rigid connection is denoted by diagonal thickening of the joint.

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RIGID FRAME

A rigid connection between two wood frame members can be obtained by nailing a plywood or OSB gusset plate on both sides of the joint. A rigid connection between two steel members is more complex.


RIGID FRAME VS. BRACED FRAME

Comparison of the racking of a rigid frame and a braced frame.


RIGID FRAME VS. BRACED FRAME

Racking of the frame is produced even under asymmetrical gravity loads, which is more pronounced in a rigid frame than in a braced frame.


MULTIBAY SINGLE-STORY FRAMES

Some of the ways of bracing a four-bay, single-story frame.



(a) Braced frames in both principal directions. (b) Braced frames in one direction and rigid frames in the other direction.



It is possible to obtain sufficient rigidity by using rigid connections in a few bays of a multibay frame.

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TALL BUILDINGS IN STEEL

Plan and elevation of a high-rise steel-frame building with a braced steel-frame shear core.


TALL BUILDINGS IN STEEL

Plan and elevation of a high-rise steel-frame building with a reinforced concrete shear core.


LONG-SPAN STEEL BUILDINGS

A commonly used method of framing a tall building that is much longer in one direction than the other is to use a central core and two end shear walls (or braced bays).

8_Ch19_Steel

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